They both could. However they could only have short careers. In a few decades after medicine for healing wounds after a fight and tech/gadgets for winning those fights improve I say they could become much more possible.
@kramotz: I do not understand why there status in the team matters when MMH and Hawkgirl were prominently featured in large roles in Justice league TAS. It didn't help them and it won't help nightwing even if he is leader. Maybe a Nightwing solo tv series would help boost his profile a bit, it kind of helped green arrow.